In current technology, fuel cell components are increasingly being integrated to simplify the stack assembly procedures and for cost savings. For example, a typical Membrane Electrode Assembly (MEA) with five layers are typically used. The MEA with five layers comprises one layer of Proton Exchange Membrane (PEM) sandwiched or disposed between two layers of dispersion mediums, with one layer of catalyst located each in the two interfaces between the PEM and the dispersion mediums. This arrangement integrates the membrane, the electrodes and the diffusion mediums and thus simplifies a stack assembly of the MEA.
However, there are other developments regarding one other significant component of a stack which is the bipolar plate.
In the PEM fuel cell industry, materials used have significantly improved. Conventional graphite bipolar plates are gradually replaced by metallic bipolar plates, which can be made relatively thin and thus, reducing weight and dimensions, as well as improving the power density of the stack. This is particularly advantageous for portable fuel cells and mobile usage.
The usage of metallic bipolar plates introduces a number of new challenges. As the metallic bipolar plates are typically made into corrugated form with at least one peak and one trough, a separator sheet is used to form air flow channels and to prevent hydrogen flow in the dispersion medium from contacting with air. The sealing of the hydrogen flow channels formed by apertures on the two ends of a stack is one challenge, particularly when corrugated sheets are used. The alignment of the corrugated sheet, the separator sheet and the MEA is another challenge.
The apertures of the constituent parts of a fuel cell, that is, a bipolar plate, a MEA, a corrugated sheet and a separator sheet are desired to be well sealed and aligned. When assembled, these parts form a continuous cylindrical shaped cavity within the fuel cell and function as an internal manifold. Hydrogen gas passes through this manifold and is distributed evenly to the respective cells. Excess hydrogen, when purged from the cells, exit through another manifold of similar arrangement.
It has been recognised that poor alignment of all the apertures within the fuel cell (i.e. between the bipolar plate, the MEA, the corrugated sheet and the separator sheet etc.) results in flow impediment of hydrogen gas. This lowers the performance of the stack due to reduced fuel intake. Additionally, poor alignment also causes non-uniform distribution of hydrogen gas to each of the cells in the stack and this results in lower overall performance due to a reduced stoichiometric number for cells with lower hydrogen intake.
It has been recognised that the apertures in both ends of the stack are not sufficient for proper alignment. As discussed, poor alignment compromises both the aesthetic appeal of the stack and also the power output performance.
In addition, the corrugated sheet used is typically coated with precious metals, such as gold or silver, and is therefore relatively expensive. However, the areas near the two ends of the stack are typically not used and thus constitute a waste of the corrugated sheet material.
Furthermore, it has been recognised that the assembly procedure of current stacks is complex due to the tight sealing requirements.
In one currently known example, MEAs are aligned within a stack assembly. The stack is an electrochemical device including a fuel cell, a compressor and an electrolyzer. A close cathode system is used in the stack assembly. Thus, a frame is used to position the MEA and separator plates. The MEA is enclosed completely in the body of the frame. Sealing arrangements, for example bead arrangement or elastomeric seals, are needed around the perimeter of the bipolar plate. Bridges are arranged at a periphery of an opening at two ends of the frame to abut against a positioning device. These requirements of sealing and bridges add to the complexity during manufacturing. In addition, strict tolerances also need to be followed during the manufacturing process to ensure that the MEA is enclosed in the frame. Furthermore, the stack assembly in the example is not secure as the frame of the bipolar plate merely rests against two positioning devices.
In another currently known example, a corrugated plate is attached to a base plate with adhesives. However, it has been recognised that when adhesives are used, there are several degrees of freedom during the manufacturing process. It is therefore typical that a tolerance limit for placement is exceeded during the manufacturing process and alignment defects appear.
In yet another currently known example, a repeater unit is used in a solid oxide fuel cell. A frame and a separator in the repeater unit establish a plurality of fuel flow path and duct houses. The ridged holes of the conduits and the separator are positioned to align a stack of the repeater units. The repeater units are firstly stacked without any secure point. Therefore, there is a problem that the stacked repeater units are typically not aligned properly.
In yet another currently known example, a bipolar plate is used in a molten carbonate fuel cell (MCFC). The MCFC is operated in a temperature higher than the melting point of electrolyte materials. The bipolar plate comprises a plurality of projection parts to adhere the bipolar plate to a masking plate to hold a current collector between the bipolar plate and the masking plate. A frame is used to position the MEA and separator plates. The MEA is enclosed completely in the body of the frame. Coupling arrangements, for example projections and penetration holes arrangement, are needed around the perimeter of the bipolar plate. This requirement of projections and penetration holes add to the complexity during manufacturing. In addition, strict tolerances also need to be followed during the manufacturing process to ensure that the MEA is enclosed in the frame.
Therefore, there exists a need for a plate member for a cell stack and a method of forming a plate member for a cell stack that seek to address at least one of the above problems.